Known guitar-type musical instruments comprise a neck, on the surface of which a board known as a "fingerboard" is applied, and a resonance body over which the instrument strings are stretched. The body can be either of the acoustic type or electronic type, in this latter case it consisting of a solid block with one or more pick-ups which are disposed in proximity to the bridge to pick up the vibrations of the individual strings and feed them to suitable amplifiers.
Each of these strings is stretched between two projections known respectively as the "bridge" and "nut", which define its useful length.
During playing of the music, player of the instrument can vary the useful or active length of each string by pressing it against the fingerboard.
Each string can thus vibrate to emit sounds of different frequencies (musical notes) as a function of its tension and useful length, and of different intensities depending on the vibration amplitude.
The instruments of the aforesaid type are divided into two categories depending on the timbre and playing requirements of the player. In this respect, known instruments are either provided or not provided with frets, such a fret being generally a metal bar disposed transversely below the strings and rigid with the upper surface of the fingerboard.
In instruments provided with frets, the player presses with his fingers a string on the other side of a fret with respect to the body, so that the string rests against said fret and assumes a useful length equal to the distance between the fret and the bridge. By choosing the correct fret, he is able to obtain from the string a discrete series of notes of precisely defined timbre, and of metallic character due to the nature of the fret. In contrast, in instruments without frets, the useful length of the string is determined by the distance between the point at which the player keeps the string pressed against the fingerboard and the bridge. In the absence of the fret, the sound assumes a much softer character. The player has therefore the possibility of choosing at his own discretion the useful length of the string with the limitation due to the predetermined position of the frets.
Basically, the two instruments with or without frets emit sounds of very different characteristics and timbre, and the same player while playing one and the same piece may require instruments with and without frets at different moments.
At the present time, in particular for base guitars, each player has two instruments available, and where the playing pattern allows it he replaces one instrument with the other according to requirements. However, this greatly limits his playing range in that the rhythm of the music does not always allow him the time necessary for making the required replacement.